The only multi-season Dexter sets ever sold legitimately are simple packaged 4-case bundles of the 4 season individual sets. The DVD set would look like THIS Some retailers have also bundled seasons 1-4, but again it's just the individual season sets packaged together. That's pretty easy information to find out before buying.

In any case, eBay and PayPal have protections for this type thing so you can get your money back.